This invention generally relates to a method of uniformly heating a part of varying thickness for hot forming and heat treatments such as hardening and tempering. More specifically, this invention relates to a method comprising both induction and conductance heating to uniformly or selectively heat a metal part having varying aspect ratios.
Metal parts of all sizes and shapes are heat treated in order to harden, strengthen, and improve properties of the parts. Heat treating to harden a metal requires heating to above a critical temperature range followed by rapid cooling, typically in oil, water, or polymer solutions. This is followed by tempering, a lower temperature reheating treatment which reduces the internal stress caused by a hardening treatment and modifies the microstructure of the metal if required.
Heat treating a metal results in changes to the microstructure of the metal. The resulting microstructure can either produce the desired hardness or can cause weaknesses in the metal. Time and temperature are critical elements in heat treating to assure that the correct microstructure is obtained. Rapid heating and cooling of the metal will produce the desired hardened metal, whereas slow heating and cooling can often result in metals that have weakened structures due to a less than optimal microstructure.
Two main methods currently used for heating metals for hardening and tempering are furnace heating and induction heating. In the former, the part to be heated is placed in a furnace and allowed to heat to the desired temperature.
This process is slow and the temperature of the metal is not easily controlled. If the part to be heated has varying aspect ratios along its dimension (i.e., varies in shape and thickness along its length), there will be undesirable differences in the rate of heating and also in temperature distribution throughout the part.
Induction heating is the heating of an electrical conducting material by eddy currents induced by a varying electromagnetic field. The metal part is placed in the center of an induction coil. As an alternating current flows through the coil, secondary currents (eddy currents) will be induced in the metal part. These eddy currents generate heat due to the metal resistivity. The advantages of induction heating over conventional processes such as furnace heating are the high speed of heating, localization of the heating, and the ease of controlling the heating to achieve the desired temperature. However, if the part to be treated varies in shape and thickness, it is difficult, or even impossible, to heat it uniformly.
Therefore, it would be desirable to have a method to uniformly heat metal parts having variable aspect ratios along the length of the part. It would be further desirable if such a method could rapidly heat the metal part. It would also be desirable if the temperature could be easily controlled during the heating process.